Semaphore signals and colour light signals.

[Condor7]

Like many on here, I was brought up on semaphore signals, and am still fortunate enough to live not far from a line that still has them.

When you are on a line that has semaphore signals, they stay down until about 5 or 10 minutes before a train is due. This means you always know when a train is due if the signal goes up. (or down in some areas).

Why then is it, that with colour light signals they return to green as soon as a section is cleared even if a train is not due for some time?

 

[ainsworth74]

Because they're automatic and operated by track circuits rather than the signaller. So if the section it's protecting is clear then it will display the appropriate aspect (be that green if the sections beyond are also clear or yellow if it not). It makes life easier for the signaller as it requires less intervention and monitoring on their part.

That being said not all colour lights are automatic plenty are still manual and only clear when operated by the signaller but the ones that you're describing where they clear even when no train is due are most likely automatic (either that or the signaller is just clearing them early). If you look at this page you'll see an example the plate that indicates that a signal is automatic.

 

[RichmondCommu]

Like many on here, I was brought up on semaphore signals, and am still fortunate enough to live not far from a line that still has them.

When you are on a line that has semaphore signals, they stay down until about 5 or 10 minutes before a train is due. This means you always know when a train is due if the signal goes up. (or down in some areas).

Why then is it, that with colour light signals they return to green as soon as a section is cleared even if a train is not due for some time?

There are colour light signals on your local line that are controlled by the signal box and will still show red until the train has been accepted by the signalman.

 

[swissrailpassion]

I think you are talking about automatic signals on main lines. These may divide up the block section allowing trains to run at close intervals in track circuit block. Basically, when the section is clear they will step up through the sequence back to green. This enables swift regulation of trains automatically. The trains regulate themselves without waiting for a signaller to react. On plain line sections it is common to find automatic signals for ease of operation. These are spaced according to line speed and braking distance to give the most efficient use of the line (in theory) At stations and junctions the colour lights will be controlled for each move by the signaller(sometimes via an automatic computer sequence). The signaller controls all signals but may only operate the essential signals, leaving the miles of plain line sections to look after themselves automatically.
There are still many locations where the colour signals will normally be at red and a change will indicate a train is due. There are many variations according to location. Some locations are resignalled with colour lights replacing semaphores but still using the existing signal levers in the box, or a small panel and absolute block working. Each location is different and varied according to it's design, method of working, date of installation and, above all, cost.

In semaphore days, when I was a boy, I too used to wait for the board to go off and then be sure a train was coming. Now, if an auto colour light has gone to red you can be sure you've just missed it.

 

[dysonsphere]

On the subject of automatic signals am I right in thinking that some cannot be overridden by a signalman at all and are only automatic. If this is correct can that lead to problems in stopping trains in an emergency.

 

[SemaphoreSam]

In the USA, where I reside, on less than busy lines the colour light signals, after train passage, go to red, then caution, then green, and after a few moments all aspects go out (saving power and bulbs, I suppose), until a train comes within a certain distance by track circuit, when the proper aspect is again displayed. Is this common practice in the UK, or are colour light signals always displayed, even during long nights with no traffic? Thanks, Sam

 

[NSEFAN]

In the USA, where I reside, on less than busy lines the colour light signals, after train passage, go to red, then caution, then green, and after a few moments all aspects go out (saving power and bulbs, I suppose), until a train comes within a certain distance by track circuit, when the proper aspect is again displayed. Is this common practice in the UK, or are colour light signals always displayed, even during long nights with no traffic? Thanks, Sam

There are some colour lights like the ones you describe in the UK, but they are quite rare I think. I have seen them in operation at Carrbridge station.

 

[Bald Rick]

On the subject of automatic signals am I right in thinking that some cannot be overridden by a signalman at all and are only automatic. If this is correct can that lead to problems in stopping trains in an emergency.

That's what radio is for.

 

[TheEdge]

In the USA, where I reside, on less than busy lines the colour light signals, after train passage, go to red, then caution, then green, and after a few moments all aspects go out (saving power and bulbs, I suppose), until a train comes within a certain distance by track circuit, when the proper aspect is again displayed. Is this common practice in the UK, or are colour light signals always displayed, even during long nights with no traffic? Thanks, Sam

What a brilliant idea.

 

[Old Hill Bank]

On the subject of automatic signals am I right in thinking that some cannot be overridden by a signalman at all and are only automatic. If this is correct can that lead to problems in stopping trains in an emergency.

On modern installations the signaller has an emergency replacement button for each of the automatic signals thus fixing the signal at red.

 

[Condor7]

What a brilliant idea.

Ditto

 

[ainsworth74]

On modern installations the signaller has an emergency replacement button for each of the automatic signals thus fixing the signal at red.

Did that come in as a result of Clapham Junction?

 

[TDK]

On modern installations the signaller has an emergency replacement button for each of the automatic signals thus fixing the signal at red.

Not all automatic signals can be keyed back by the signaller, only selected ones can, there is a ratio but I do not know what is is, maybe someone into S & T can enlighten us.

 

[RPM]

In the USA, where I reside, on less than busy lines the colour light signals, after train passage, go to red, then caution, then green, and after a few moments all aspects go out (saving power and bulbs, I suppose), until a train comes within a certain distance by track circuit, when the proper aspect is again displayed. Is this common practice in the UK, or are colour light signals always displayed, even during long nights with no traffic? Thanks, Sam

The only UK example of this that I know is on the LUL infrastructure at Rickmansworth. There is a signal that controls a wrong direction movement into the up platform and it shows no aspect unless the preceding track circuit is occupied.

 

[Old Hill Bank]

Not all automatic signals can be keyed back by the signaller, only selected ones can, there is a ratio but I do not know what is is, maybe someone into S & T can enlighten us.

Certainly back in the 1960s when all of the West Midlands PSBs were built your statement would be true, I believe it was something like every fifth signal on any line of route had to be either a controlled or semi-automatic and therefore able to be maintained at red by the signaller.

As for more modern installations (as someone who worked on re-signalling projects until a couple of years ago) I stand by my earlier statement.

 

[Tomnick]

Not all automatic signals can be keyed back by the signaller, only selected ones can, there is a ratio but I do not know what is is, maybe someone into S & T can enlighten us.

It used to be 1 in 5, I think, but nowadays the relative ease of providing replacement facilities means that new installations often (always?) have the ability to replace all auto signals.

 

[broadgage]

Another reason for semaphore signals showing "stop" until shortly before a train is expected is that in extreme weather the signal may freeze or be jammed by driven snow. It is clearly preferable to show a false "stop" signal or a false caution in the case of a distant, than a false proceed signal.
The former merely causes delay, the later could result in a serious accident.
If the signal is normly in the danger position, then any freezing or jamming is more likely to hold it thus, rather in the proceed position.

In the case of colour light signals, most types use a bulb for each aspect. These are normaly replaced long before failure so as to ensure reliabilty.
Nevertheless, some do fail, and it is considered preferable to show a green except when yellow or red is actually required.
This means that the bulb in the red aspect gets the least use and is less liable to failure than the green.

The failure of the bulb in the red aspect can have very serious consequences. Although the rules require that train drivers treat unlit or otherwise defective signals as being at danger, on a dark night the driver might not see the unlit signal and proceed into danger.
The actual risk is very small because the lamps have two filaments, and any failure often puts the previous signal back to danger. Also a red normally follows a yellow caution signal and a driver should be slowing and expecting a red.
But ensuring that green lamps normaly get more use than the red does furthur reduce the already small risks.

Failure of the lamp for the green aspect is unlikely to have any serious results. Either the driver notices and makes a needless but safe emergency stop, or the driver does not notice it and proceeds as normal, which is what the signal would have indicated anyway.

 

[matchmaker]

The only UK example of this that I know is on the LUL infrastructure at Rickmansworth. There is a signal that controls a wrong direction movement into the up platform and it shows no aspect unless the preceding track circuit is occupied.

As stated above some of the signals on the Highland main line are switched off when not required. Slochd Summit is an example. They are all on the northern section of the line which (apart from Aviemore station) is colour light.

They are on loops which are signalled bi-directionally. I think the reason is so that a driver who is passing on the main line, rather than the loop, is not faced with a red signal applying to the same direction.

Another example is at Alloa East.

 

[Tomnick]

Nevertheless, some do fail, and it is considered preferable to show a green except when yellow or red is actually required.
This means that the bulb in the red aspect gets the least use and is less liable to failure than the green.

Sounds like a bit of a myth to me. There are plenty (a majority?) of controlled signals where a red's shown for most of the time; automatic signals (or controlled signals with an auto button) will indeed show a proceed aspect 'by default', but this is as a direct consequence of the way that they're operated solely by track circuit operation (in turn, vastly reducing the Signalman's workload by avoiding the need to pull off right through for each train individually) rather than the need to avoid showing a red too much.

 

[Railsigns]

Sounds like a bit of a myth to me. There are plenty (a majority?) of controlled signals where a red's shown for most of the time; automatic signals (or controlled signals with an auto button) will indeed show a proceed aspect 'by default', but this is as a direct consequence of the way that they're operated solely by track circuit operation (in turn, vastly reducing the Signalman's workload by avoiding the need to pull off right through for each train individually) rather than the need to avoid showing a red too much.

Indeed. The most critical stop signals are those which protect conflicts at junctions or prevent head-on collisions; such signals must be controlled, not automatic, and so usually display a danger aspect. Automatic signals are less critical since passing one of those at danger can only result in a rear-end collision with the train in front, hence why it's extremely rare to find TPWS fitted at an automatic signal.

 

[Maxfly]

As stated above some of the signals on the Highland main line are switched off when not required. Slochd Summit is an example. They are all on the northern section of the line which (apart from Aviemore station) is colour light.

They are on loops which are signalled bi-directionally. I think the reason is so that a driver who is passing on the main line, rather than the loop, is not faced with a red signal applying to the same direction.

Another example is at Alloa East.

Another is LU2 controlled by Lugton, this only illuminates when the route is set towards it. Grab catch handle for LU1 and untill LU2 lights up LU1 won't realease from the frame. Again bi-di signalling and your right in that it is 'approach lit' for drivers on the other line not having to be confused.

This is on the GBK line.

 

[Railsigns]

Around the 1930s, British colour light signals in outlying areas (typically intermediate block signals) were often approach lit to conserve battery power. This became unnecessary with the arrival of reliable mains power supplies.

Currently there are about 30 approach lit signals on Network Rail infrastructure, all but one of which are in Scotland (the other is at Haresfield, near Gloucester). These exist for sighting reasons on loops or other secondary lines, where there is no signal at the same position for the adjacent main line.

From next weekend there will be two new approach lit signals on the East Kilbride line and from May, a new one at Midcalder Junction.

 

[t o m]

On the subject of automatic signals am I right in thinking that some cannot be overridden by a signalman at all and are only automatic. If this is correct can that lead to problems in stopping trains in an emergency.

There are signal post replacement switches at automatic signals that the S&T can use to put the signal to red with the use of a special key.
--- old post above --- --- new post below ---

In the case of colour light signals, most types use a bulb for each aspect...

You mean a lamp, bulbs go in the ground

 

[SemaphoreSam]

Another reason for semaphore signals showing "stop" until shortly before a train is expected is that in extreme weather the signal may freeze or be jammed by driven snow. It is clearly preferable to show a false "stop" signal or a false caution in the case of a distant, than a false proceed signal.
The former merely causes delay, the later could result in a serious accident.
If the signal is normly in the danger position, then any freezing or jamming is more likely to hold it thus, rather in the proceed position.

The classic semaphore case of a 'wrong side failure' being Abbots Ripton, in '76, where the signal failed (due to ice and snow) to an aspect indicating the block was clear, and traffic could proceed. Another 'wrong side failure' was Clapham Junction, where due to a stray wire supplying current to a circuit, a colour light showed 'green' when the block was occupied. These are, obviously, extremely dangerous. Sam

 

[455driver]

It used to be 1 in 5, I think, but nowadays the relative ease of providing replacement facilities means that new installations often (always?) have the ability to replace all auto signals.

Automatic signals are no longer being installed!

Well if you look at the Basingstoke area all the signals that replaced the automatic ones are plated as manual even though they work exactly the same as autos.
--- old post above --- --- new post below ---

Currently there are about 30 approach lit signals on Network Rail infrastructure, all but one of which are in Scotland (the other is at Haresfield, near Gloucester). These exist for sighting reasons on loops or other secondary lines, where there is no signal at the same position for the adjacent main line.

That is interesting, we have several loops where the loop has a signal but the through line doesnt, none of them are approach lit, it is down to the drivers route knowledge to remember and has never caused a problem, note linespeeds are up to 70mph.

 

[Mojo]

The only UK example of this that I know is on the LUL infrastructure at Rickmansworth. There is a signal that controls a wrong direction movement into the up platform and it shows no aspect unless the preceding track circuit is occupied.

As stated above some of the signals on the Highland main line are switched off when not required. Slochd Summit is an example. They are all on the northern section of the line which (apart from Aviemore station) is colour light.

They are on loops which are signalled bi-directionally. I think the reason is so that a driver who is passing on the main line, rather than the loop, is not faced with a red signal applying to the same direction.

Another example is at Alloa East.

Also elsewhere on the Underground is signal WD13[sup]A[/sup] in the Hammersmith/Barons Court central siding. The colour light signal only illuminates once a train is signalled into the siding from the west end. It is accompanied by shunt signal WD13[sup]B[/sup] which is always visible on account of it being a rotating disc and not fibre-optic.

 

[Tomnick]

Automatic signals are no longer being installed!

Well if you look at the Basingstoke area all the signals that replaced the automatic ones are plated as manual even though they work exactly the same as autos.

I don't know whether that's been done anywhere else since...I suspect it was a rather short-lived idea, thankfully!

Just as an interesting (maybe) aside, there are one or two older examples of auto signals being plated as controlled though (to prevent Drivers passing it on their own authority - is this what they were getting at on the Basingstoke scheme?), and a couple of examples of controlled signals plated as autos (where they used to protect a long-since-removed connection of some sort, and have spent their life since then working in auto).

The distinction's irrelevant nowadays anyway, of course, thanks to the latest effort to confuse us all .

 

[455driver]

Just as an interesting (maybe) aside, there are one or two older examples of auto signals being plated as controlled though (to prevent Drivers passing it on their own authority - is this what they were getting at on the Basingstoke scheme?),

It is.

The distinction's irrelevant nowadays anyway, of course, thanks to the latest effort to confuse us all .

Dont we know it, still we get another load of "updates" in June! :roll:

Is it me or does the AM module contain more (relevant) info than the rest of the rule book?

 

[AndyPJG]

Automatic signals are no longer being installed!

Well if you look at the Basingstoke area all the signals that replaced the automatic ones are plated as manual even though they work exactly the same as autos.

Automatic signals with emergency replacement won't be plated as autos, so that a driver can't pass on his/her own authority as the signaller may be holding the signal at danger for a good reason.

It used to be that when not all autos had ERs, those that did could often be protecting LXs or tunnels.

 

[Tomnick]

The AM module's nowt but a nuisance! I do think I'd be rather good at Origami now though... in places, I've got amendments stuck over amendments, all nicely folded 'cos there's not enough space on the original page!

AndyPJG - I'm sure there are plenty of auto signals (with ERSs) that are plated as such?